1. Field of the Invention
The present invention relates to a mobile communication antenna device and, more particularly, to a mobile communication antenna device mounted in compact mobile terminal equipment including portable radio calling units, transceivers, cordless telephones, portable telephones, portable information terminals, and the like.
2. Description of the Prior Art
Recently, mobile terminal equipment, in particular, has rapidly improved in performance and decreased in size. In contrast to this, since mobile communication antenna devices basically tend to have good characteristics with respect to wavelengths as they increase in size, the miniaturization of the devices is limited.
Provided that the size and function of an antenna remain the same, the band narrows as the gain increases, and vice versa. That is, the gain and the band tend to contradict each other. For this reason, as the size of an antenna is decreased, either or both of the gain and the band are sacrificed, resulting in a deterioration in characteristics. FIG. 1 is a graph showing the correlation between the antenna size and the antenna gain/band.
In consideration of such antenna characteristics, compact, high-performance antennas such as inverted F antennas, microstrip antennas, and whip antennas have been used for mobile terminal equipment. A loop antenna as a magnetic field type antenna, is used for a radio selective calling receiver, generally called a pager, to obtain a high gain while the unit is carried by the user in close proximity to his/her body.
As described above, to obtain good antenna characteristics, the antenna size is preferably increased. With the tendency toward a reduction in size of an equipment body, however, a sufficient space for an antenna is difficult to ensure.
For example, FIG. 2 shows a structure proposed to solve the problem associated with the space for an antenna. In this structure, a loop antenna 81 is mounted in an equipment housing 80 by using its inner wall side. Referring to FIG. 2, reference numeral 82 denotes a storage portion which is a recess portion in which a battery is to be mounted. In contrast to this, in the antenna device for the electronic device disclosed in Japanese Unexamined Utility Model Publication No. 2-70510, the antenna member is slidably mounted on the battery cover detachably mounted on the housing of the electronic device body.
In the conventional structure shown in FIG. 2, however, when the loop antenna 81 is mounted in the housing 80, it is basically difficult to completely prevent interference between the antenna and other electronic parts mounted in the housing. In the antenna device disclosed in Japanese Unexamined Utility Model Publication No. 2-70510, since antenna characteristics are uniquely determined by the structure in which the antenna slidably moves on the surface of the battery cover, it is difficult for the antenna device to cope with various types of electronic devices demanding various levels of performance. That is, it is structurally impossible to greatly improve the antenna characteristics of the electronic device for which this antenna device is used.